High contrast photographic elements find particular utility in the graphic arts in which images are recorded in the form of half-tone dots. Exposure is conducted in a camera through a half-tone screen. The original is illuminated on the copy board of the camera by a high intensity light source such as pulsed xenon or quartz-iodine. A high photographic contrast is a requirement for accurate recording of half-tone images where it is desirable that exposure will generate either a full response or zero response.
Photographic elements for laser scanner imaging are designed to be imaged by electronically-modulated high resolution raster scanners, which scan the film with a very small spot of light from a high intensity source. Examples of high intensity sources include (i) gas lasers, especially argon ion, emitting at 488 nm, helium-neon, emitting at 633 nm, or helium-cadmium, emitting at 442 nm, (ii) near-infrared (NIR) laser diodes, which may emit light in the range 750-1500 nm, and (iii) light-emitting diodes (LED), which may emit in either the visible or NIR range. In all cases, the spot is scanned very rapidly, so that the dwell time on any part of the photographic element is short, typically from 10.sup.-7 to 10.sup.-6 seconds.
Silver halide photographic films usually respond optimally to exposures of duration of from 1 to 100 milliseconds, and tend to perform relatively badly under microsecond exposures, losing up to 1.0 logE in speed and 50% in average contrast. This is due to the phenomenon of high intensity reciprocity failure (HIRF), which also gives rise to related problems, such as:
(i) intermittency effects, which cause multiple superimposed short exposures to have a progressively greater effect as the time interval separating them is increased from microseconds to milliseconds or longer, PA1 (ii) latent image progression, whereby the latent image gives a stronger developed image when there is a delay period, especially of up to 1 hour, between exposure and development, PA1 (iii) unusually high sensitivity to development conditions, e.g. state of exhaustion of the developer.
It is desirable to overcome all these problems by making a photographic element which does not suffer from HIRF and thus responds equally to any given amount of exposure, regardless of how short or fragmented a form in which the exposure may be delivered.
It is known to prepare photographic emulsions containing small quantities of some Group VIII noble metal compounds. These metal compounds impart different properties to the emulsions, some compounds reduce HIRF and others may increase contrast For example, U.S. Pat. Nos. 3,790,390 and 4,147,542 disclose photographic emulsion containing at least one compound belonging to Group VIII together with particular sensitising dyes. Such dopants are advantageously added during the crystal growth stages of emulsion preparation, i.e. during initial precipitation, and/or during physical ripening of the silver halide crystals. Halide compounds of rhodium and iridium are the dopants most commonly used in this way. When such dopants are incorporated into conventional, negative working photographic emulsions, certain specific photographic effects are obtained, depending on the particular compound employed.
For example, hexachloroiridate complex salts of formula M.sub.3 IrCl.sub.6 or M.sub.2 IrCl.sub.6 (where M is a Group I metal), are incorporated as emulsion dopants with consequent improvement in sensitivity to high intensity exposure, and reduction in the desensitisation usually caused by mechanical stress. This phenomenon is disclosed, for example, in British Pat. Nos. 1 527 435 and 1 410 488, U.S. Pat. Nos. 4,126,472 and 3,847,621, German Pat. DE No. 3 115 274, and French Pat. No. 2 296 204.
The action on silver halide emulsions of halide compounds of rhodium is altogether different. These compounds produce the effect of increasing the contrast of the developed image, together with overall desensitisation of the emulsion. Rhodium doping is disclosed in a number of patents, e.g. rhodium trichloride in British Pat. No. 775 197, sodium hexachlororhodate in British Pat. No. 1 535 016; potassium hexachlororhodate in British Pat. No. 1 395 923; ammonium hexachlororhodate (III) in British Pat. No. 2 109 576 and U.S. Pat. No. 3,531,289, and rhodium chloride or trichloride in German Pat. Nos. DT 2 632 202A, DE 3 122 921 and Japanese Application No. 74-33781.
Silver halide emulsions doped with Group VIII metal compounds suffer from the disadvantage of instability of speed and contrast upon ageing. U.S. Pat. No. 3,488,709 discloses the addition of cadmium salts to rhodium containing silver halide emulsions as a stabilizer. Japanese Pat. Publication No. 52-18310 discloses stable silver halide emulsions containing rhodium salts in combination with spectral sensitizing dyes having an oxidation potential (Eox) greater than 0.79V. It is stated that the oxidation potentials of spectral sensitising dyes cannot be inferred from similarity of their structural formula. For example even if only one substituent is different, the oxidation potentials may differ considerably. The art therefore does not provide any indications of which types of organic molecules are liable to be useful as spectral sensitising dyes.
We have now found a class of structurally related compounds which are powerful sensitisers in silver halide emulsions doped with Group VIII metal compounds and impart good stability properties to the emulsion. In particular the dyes have proved useful with silver halide containing diffusion transfer printing plates and silver halide emulsions suitable for laser exposure.